In 1994 we discovered that yeast can have prions, infectious proteins analogous to the transmissible spongiform encephalopathies of mammals. We showed that the non-mendelian genetic element, URE3, is a prion of the Ure2 protein, and that PSI+ is a prion of Sup35p (1). We found the first biochemical evidence confirming our discovery (2) and defined the prion domain of Ure2p (2). These prions are amyloids of the respective protein (reviewed in 3). Unexpectedly, shuffling the prion domain amino acid sequence of Ure2p or Sup35p did not alter the ability of these domains to support prion formation, suggesting that the amyloid structure is parallel in-register (4-6). We have shown by solid-state NMR (in collaboration with Rob Tycko of NIDDK) that the amyloids of Ure2p, Sup35p and Rnq1p are indeed in-register parallel beta sheets (7-10). It has not escaped our notice that this in-register parallel beta sheet structure can explain how a given protein sequence can encode any of several biologically distinct prion variants based on biochemically distinct self-propagating amyloid structures (11,12). We have recently discovered a new prion, MCA, based on a self-propagating amyloid of Mca1p, a metacaspase homolog (13). This prion was discovered based on a general search from a gene bank, a method that we are now trying to apply to other organisms. We are currently studying the mechanism of toxicity of this prion and the structure of the infectious amyloid formed by Mca1p. We have examined the URE3 prions based on Ure2 proteins from non-cerevisiae species of Saccharomyces, and have demonstrated a similar species barrier to that seen among mammals of different species in their transmission of spongiform encephalopathies (14). We showed that the variant properties, as defined by species barrier, are maintained even during passage through a different species. We also noted that the Ure2p of Saccharomyces castellii cannot become a prion (14). Recently, we have examined the properties of several non-prion amyloids that have normal functions for their respective organisms: Pmel17, a mammalian melanocyte protein that normally forms amyloid involved in melanin biosynthesis and curli, a cell-surface amyloid of E. coli that is involved in adhesion. We found that the repeat domain of Pmel17 is able to form an amyloid structure that forms readily at the mildly acid pH typical of the melanocyte, and rapidly dissolves at neutral pH (15). We characterized the amyloid form by curli proteins and found that they are not in the in-register parallel architecture typical of the yeast prion amyloids (16). In collaborative efforts, we participated in a study showing that the Sup35NM (prion domain) can propagate as a prion in animal cells in tissue culture (17). In another collaboration, we participated in the development of a method for determining mass per length of amyloid filaments using dark field with the usual transmission electron microscope instead of the less readily available STEM (18). 1. Wickner RB (1994) URE3 as an altered URE2 protein: evidence for a prion analog in S. cerevisiae. Science 264: 566 - 569. 2. Masison DC &Wickner RB (1995) Prion-inducing domain of yeast Ure2p and protease resistance of Ure2p in prion-containing cells. Science 270: 93 - 95. 3. Wickner RB, Edskes HK, Ross ED, Pierce MM, Baxa U, Brachmann A &Shewmaker F (2004) Prion Genetics: New Rules for a New Kind of Gene. Ann. Rev. Genetics 38: 681-707. 4. Ross ED, Baxa U &Wickner RB (2004) Scrambled prion domains form prions and amyloid. Mol Cell Biol 24: 7206-7213. 5. Ross ED, Edskes HK, Terry MJ &Wickner RB (2005) Primary sequence independence for prion formation. Proc Natl Acad Sci U S A 102: 12825 - 12830. 6. Ross ED, Minton AP &Wickner RB (2005) Prion domains: sequences, structures and interactions. Nat. Cell Biol. 7: 1039-1044. 7. Shewmaker F, Wickner RB &Tycko R (2006) Amyloid of the prion domain of Sup35p has an in-register parallel b-sheet structure. Proc. Natl. Acad. Sci. USA 103: 19754 - 19759. 8. Baxa U, Wickner RB, Steven AC, Anderson D, Marekov L, Yau W-M &Tycko R (2007) Characterization of b-sheet structure in Ure2p1-89 yeast prion fibrils by solid state nuclear magnetic resonance. Biochemistry 46: 13149 - 13162. 9. Wickner RB, Dyda F &Tycko R (2008) Amyloid of Rnq1p, the basis of the PIN+ prion, has a parallel in-register b-sheet structure. Proc Natl Acad Sci U S A 105: 2403 - 2408. 10. Shewmaker F, Kryndushkin D, Chen B, Tycko R &Wickner RB (2009) Two prion variants of Sup35p have in-register b-sheet structures, independent of hydration. Biochemistry 48: 5074-5082. 11. Wickner RB, Shewmaker F, Kryndushkin D &Edskes HK (2008) Protein inheritance (prions) based on parallel in-register b-sheet amyloid structures. Bioessays 30: 955 - 964. 12. Wickner RB, Shewmaker F, Edskes H, et al. (2010) Prion amyloid structure explains templating: how proteins can be genes. FEMS Yeast Res. 10: Jul. 16 epub ahead of print. 13. Nemecek J, Nakayashiki T &Wickner RB (2009) A prion of yeast metacaspase homolog (Mca1p) detected by a genetic screen. Proc. Natl. Acad. Sci. USA 106: 1892 - 1896. 14. Edskes HK, McCann LM, Hebert AM &Wickner RB (2009) Prion variants and species barriers among Saccharomyces Ure2 proteins. Genetics 181: 1159 - 1167. 15. McGlinchey RP, Shewmaker F, McPhie P, Monterroso B, Thurber KR &Wickner RB (2009) The repeat domain of the melanosome fibril protein Pmel17 forms the amyloid core promoting melanin synthesis. Proc. Natl. Acad. Sci. USA 106: 13731-6. 16. Shewmaker F, McGlinchey R, Thurber KR, McPhie P, Dyda F, Tycko R &Wickner RB (2009) The functional curli amyloid is not based on in-register parallel beta-sheet structure. J. Biol. Chem. 284: 25065 - 25076. 17. Krammer C, Kryndushkin D, Suhre MH, et al. (2009) The yeast Sup35NM domain propagates as a prion in mammalian cells. Proc. Natl. Acad. Sci. USA 106: 462 - 467. 18. Chen B, Thurber KR, Shewmaker F, Wickner RB &Tycko R (2009) Measurement of amyloid fibril mass-per-length by tilted-beam transmission electron microscopy. Proc. Natl. Acad. Sci. USA 106: 14339 - 14344.